There are a variety of head gaskets known in the prior art. Head gaskets generally comprise thin flat bodies which define a plurality of apertures, including oil and water openings and cylinder openings.
In high-compression diesel engines operating at high temperatures and high pressures, it has been a practice for a number of years now to provide head gaskets with armoring adjacent the cylinder or combustion openings to seal the cylinder openings and to protect the remainder of the gasket from the effects of the high temperature and pressure. Early armoring covered the entire gasket surface as well as the cylinder openings, as typified by U.S. Pat. No. 1,819,694.
It was found subsequently that armor embracing the edges of the gasket body adjacent the cylinder opening was sufficient to protect the gasket and to seal the head and block at the cylinder bore. Further, armor of this general type has been provided with attaching sections and such a construction is illustrated in British Patent Specification No. 1,441,505. It has also been suggested that head gasket armor be positioned in a combustion opening and be suspended from tabs which project outwardly of the combustion opening and into engagement with the gasket body, and a head gasket of this type is illustrated in U.S. Pat. No. 3,565,449.
More recently, diesel engines have been introduced into automobiles in increasing numbers. In typical diesel engines used in the United States, it has become a practice to provide a precombustion chamber for each cylinder bore which feeds fuel into the cylinder bore and to position that precombustion chamber (frequently referred to as a pre-cup) in the head and straddling the armoring. To insulate the gasket, to effect a seal, and to provide a seal for the pre-cup, a conventional diesel gasket, such as of the type shown in U.S. Pat. No. 3,565,449, has been used with an expansive tab (a pre-cup or precombustion tab) extending from the armor and outwardly over the main gasket body. Such a tab is illustrated in U.S. Pat. No. 4,311,318.
It was found that pre-cup tab modified gaskets tended to permit blow-by, especially when the engine was cold, and as a result, the associated gaskets too frequently failed. Worse, the blow-by sometimes communicated with the oil and water ports, causing blow-out of the oil or water, the cooling media for the engine, which sometimes caused the engine itself to burn out.
It was determined that one of the possible reasons for the failure of the head and pre-cup to be sealed by the armoring, as a head without a pre-cup will usually do, is the discontinuity at the intersection between the head and pre-cup where it crosses or intersects the armor in the zone in which the wire of the armor underlies the armor sheath. For example, if the pre-cup projects beyond the plane of the head, then at the intersection with the wire, the wire is placed under a greater compressive load by the pre-cup than by the head. As a result, at the intersection there is a zone which may not force the ensheathing armor into sealing engagement with the head, the projecting edge of the pre-cup and then the pre-cup bottom surface. The same result could occur if the pre-cup was recessed, or if there was a significant chamfer at the surface of the bore receiving the pre-cup. In that case there would be a zone in which the armor did not adequately seal the confronting head and pre-cup surfaces, and therefore there would be provided a possible path for the escape of high pressure gases from the combustion cylinder, with the attendant damage these gases could inflict.
One solution to the problem was to grind the faces of the pre-cup and head to a common plane and to make absolutely certain there is no chamfer at the head or pre-cup surface. However, that is impractical and expensive, and is not within the control of gasket manufacturers. Another solution was to assemble parts so that they were absolutely flush. However, that is not a practical solution either, for tolerances of even several thousandths of an inch could not be permitted and holding to zero tolerances for pairs of mass-produced assembled parts is extremely difficult and exceedingly expensive.
Neither was it possible simply to increase the head load or to use a thicker or softer wire ring because available head loads are limited, are normally used up to their maximum in engines of the type under consideration, and the load distribution in the zones of the armor and main body of the gasket are calculated, determined and balanced by design. To substantially increase the portion of the load taken up in the area of the armor would reduce the load availabe in the main body and would then tend to permit leakage there.
One practical solution was found to be the addition of spanner means underlying one of the legs of the armor at each of the two spaced zones of intersection of the pre-cup and the armor, so that when the gasket assembly was disposed between the head and block, and was placed under load, a more effective and consistent seal between the armor adjacent the precombustion tab and the head, and a precombustion chamber was obtained. That solution is disclosed in U.S. Pat. No. 4,311,318 wherein spanner means may be integrally formed with the metallic armor annulus and may be reversely folded to underlie the wire ring, and wherein the spanner means preferably comprises a pair of reversely folded spaced spanner elements which are integrally formed with the metallic annulus and which are preferably disposed between the annulus leg spaced away from the precombustion tab and the wire ring.